My school has the same rule, But they don't let us use the Office phones, we have to pay. and the pay phones are always super dirty.
I went in the office on friday and asked to call my mom, I siriously felt like fainting..And she asked, Do you have money. I was like "Huh" and she said too bad. So i went to my locker and called my mom from my cell.
I din't really care if i got in trouble because it wasent my fault, they wouldent let me use the office phone.